Digital accessory interface calibration

ABSTRACT

A method of compensating for a round-trip transmission delay in an audio system comprising: a portable communications device; an audio accessory; and a cable, suitable for connecting the portable communications device and the audio accessory, having at least one wire and being detachable from at least one of the portable communications device and the audio accessory. At first times, a first synchronization data pattern is transmitted on the at least one wire from said device to said accessory, wherein said first synchronization data pattern comprises first signal level transitions on the at least one wire, synchronized to a master transmission clock. At second times, a second synchronization data pattern is transmitted on the at least one wire from said accessory to said device, wherein said second synchronization data pattern comprises second signal level transitions at timings that are set based on a delay value stored in said accessory. In the device, timings of said second signal level transitions as received at said device are determined relative to the master transmission clock, and timing delay control data based on said determination is transmitted from the device to the accessory. In the accessory, the timing delay control data is received, and the stored delay value is updated based on the timing delay control data.

FIELD OF DISCLOSURE

This invention relates to a method of transferring data over a wired communications link. In particular, it relates to an interface that is suitable for example for transferring digital data between a host device and an accessory that is connected by a removable cable to the host device.

BACKGROUND

When connecting an accessory, such as a headset with at least one microphone and at least one speaker, to a host device such as a mobile phone, it is generally required to transmit signals from the host device to the accessory and from the accessory to the host device.

Since many existing host and accessory devices are equipped with standard 4-pin sockets or jacks, there is an advantage in providing an interface that can use a total of four wires for the connection. Since one of the wires is used for a ground connection, there are only three signal conductors available. It is advantageous to use one of these conductors to provide power from the host device to the accessory, to avoid the need to provide a battery in the accessory, and so only two conductors are available for transferring the signals from the host device to the accessory and from the accessory to the host device.

There are known advantages in using balanced transmission, which for example reduces electromagnetic interference (EMI) emissions and improves sensitivity, but this requires the use of two wires per communication path, rather than one.

These factors mean that it may be preferable to provide a bidirectional (half duplex) link between the host device and the accessory on one wire (or on one pair of wires), rather than providing a full duplex link that would have a connection from the host device to the accessory on one wire, and from the accessory to the host device on the other wire.

One issue that arises with a bidirectional link on a single wire is the need to take account of the transmission delay between the host device and the accessory. For example, when one data symbol is transferred in a first direction from a first device to a second device, it arrives at the second device after a certain time delay. Then, if the requirement is that the next data symbol should be transmitted from the second device to the first device, there is a further transmission delay before that next data symbol is received by the first device.

In the case of a host device and an accessory, the same host device needs to be usable with different accessories, while one accessory needs to be usable with different hosts. Therefore, it is useful for the devices to be able to take account of the transmission delay that applies for a particular combination of host device and accessory.

SUMMARY

According to a first aspect of the present invention, there is provided a method of compensating for a round-trip transmission delay in an audio system comprising:

-   -   a portable communications device;     -   an audio accessory; and         a cable, suitable for connecting the portable communications         device and the audio accessory, having at least one wire and         being detachable from at least one of the portable         communications device and the audio accessory,     -   the method comprising:         at first times, transmitting on the at least one wire, from said         device to said accessory, a first synchronisation data pattern,         wherein said first synchronisation data pattern comprises first         signal level transitions on the at least one wire, synchronised         to a master transmission clock;         at second times, transmitting, on the at least one wire, from         said accessory to said device, a second synchronisation data         pattern, wherein said second synchronisation data pattern         comprises second signal level transitions at timings that are         set based on a delay value stored in said accessory;         in said device, determining, relative to the master transmission         clock, timings of said second signal level transitions as         received at said device;         transmitting, from said device to said accessory, timing delay         control data based on said determination;     -   in said accessory, receiving said timing delay control data; and     -   updating said stored delay value based on said timing delay         control data.

The method may comprise:

-   -   repeatedly iterating the steps of transmitting the second         synchronisation data pattern, determining timings of said second         signal level transitions, transmitting timing delay control         data, receiving said timing delay control data, and updating         said stored delay value based on said timing delay control data,         until a termination condition is met.

The method may comprise:

-   -   in said device, setting a desired timing relative to the master         transmission clock; and     -   determining whether said timings of said second signal level         transitions as received at said device are early or late         relative to the desired timing.

The method may comprise:

-   -   transmitting, from said device to said accessory, timing delay         control data to retard or advance the timings of the second         signal level transitions at said accessory, depending on whether         said timings of said second signal level transitions as received         at said device are early or late relative to the desired timing.

Said timing delay control data may be adapted to retard or advance the timings of the second signal level transitions at said accessory by a known step size. Said known step size may be varied from step to step.

The method may comprise halving said known step size each time that the determination as to whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing produces a result that is opposite to an immediately preceding iteration.

Said timing delay control data transmitted from the device to the accessory may directly represents the delay to be applied within the accessory, or may comprise an indication of the step size and polarity by which the stored delay value needs to be increased or decreased, or may comprise an indication of whether the delay needs to be increased or decreased.

The method may comprise determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing a signal level at the desired timing with an expected signal level at the desired timing, or may comprise determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing an average signal level at the desired timing, obtained over multiple transitions, with the expected signal level at the desired timing, or may comprise determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by detecting whether signal levels at the desired timing represent logic 0's or logic 1's, and comparing the numbers of logic 0's or logic 1's obtained over multiple transitions, in which case it may comprise comparing a proportion of logic 0's and 1's with a 50:50 split.

Said desired timing relative to the master transmission clock may provide a predetermined time margin at the device between an end of a received symbol and a start of a transmitted symbol.

The method may further comprise, after performing said method, entering an operational mode.

The method may comprise in the operational mode, in each of a plurality of frames:

-   -   transmitting a synchronisation data pattern from said device to         said accessory;     -   transmitting first payload data from said device to said         accessory;     -   transmitting second payload data from the accessory to said         device; and     -   transferring control data between the device and the accessory,     -   wherein transmissions from the device are synchronised to the         master transmission clock,     -   wherein transmissions from the accessory are at timings         determined by the stored delay value, and     -   wherein the format of the frame is such that, irrespective of         whether the control data is transmitted from the device to the         accessory or transmitted from the accessory to the device, there         is only one pair of reversals of a direction of data transfer in         each frame. A plurality of said frames may form a superframe,         and control data in the frames of a superframe may form a         plurality of control words.

According to a second aspect of the present invention, there is provided a method of compensating for a round-trip transmission delay in an audio system comprising:

-   -   a portable communications device;     -   an audio accessory; and         a cable, suitable for connecting the portable communications         device and the audio accessory, having at least one wire and         being detachable from at least one of the portable         communications device and the audio accessory,     -   the method comprising, in the portable communications device:         at first times, transmitting on the at least one wire first         synchronisation data pattern, wherein said first synchronisation         data pattern comprises first signal level transitions on the at         least one wire, synchronised to a master transmission clock;         at second times, receiving from said accessory a second         synchronisation data pattern, wherein said second         synchronisation data pattern comprises second signal level         transitions;         determining, relative to the master transmission clock, timings         of said second signal level transitions as received at said         device; and         transmitting to said accessory timing delay control data based         on said determination.

The method may comprise:

-   -   repeatedly iterating the steps of receiving the second         synchronisation data pattern, determining timings of said second         signal level transitions, and transmitting timing delay control         data, until a termination condition is met.

The method may comprise:

-   -   setting a desired timing relative to the master transmission         clock; and     -   determining whether said timings of said second signal level         transitions as received at said device are early or late         relative to the desired timing.

The method may comprise:

-   -   transmitting to said accessory timing delay control data to         retard or advance the timings of the second signal level         transitions transmitted from said accessory, depending on         whether said timings of said second signal level transitions as         received at said device are early or late relative to the         desired timing. Said timing delay control data may be adapted to         retard or advance the timings of the second signal level         transitions transmitted from said accessory by a known step         size. Said known step size may be varied from step to step.

The method may comprise halving said known step size each time that the determination as to whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing produces a result that is opposite to an immediately preceding iteration.

The method may comprise determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing a signal level at the desired timing with an expected signal level at the desired timing, or may comprise determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing an average signal level at the desired timing, obtained over multiple transitions, with the expected signal level at the desired timing, or may comprise determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by detecting whether signal levels at the desired timing represent logic 0's or logic 1's, and comparing the numbers of logic 0's or logic 1's obtained over multiple transitions, in which case it may comprise comparing a proportion of logic 0's and 1's with a 50:50 split.

Said desired timing relative to the master transmission clock may provide a predetermined time margin at the device between an end of a received symbol and a start of a transmitted symbol.

The method may further comprise, after performing said method, entering an operational mode.

The method may comprise, in the operational mode, in each of a plurality of frames:

-   -   transmitting a synchronisation data pattern from said device to         said accessory;     -   transmitting first payload data from said device to said         accessory;     -   receiving second payload data from the accessory; and     -   transferring control data between the device and the accessory,     -   wherein transmissions from the device are synchronised to the         master transmission clock, and     -   wherein the format of the frame is such that, irrespective of         whether the control data is transmitted from the device to the         accessory or transmitted from the accessory to the device, there         is only one pair of reversals of a direction of data transfer in         each frame.

A plurality of said frames may form a superframe, and control data in the frames of a superframe may form a plurality of control words.

According to a third aspect of the present invention, there is provided a portable communications device, configured for forming part an audio system further comprising:

-   -   an audio accessory; and         a cable, suitable for connecting the portable communications         device and the audio accessory, having at least one wire and         being detachable from at least one of the portable         communications device and the audio accessory, the portable         communications device being adapted to compensate for a         round-trip transmission delay between the portable         communications device and the audio accessory by performing the         method according to any embodiment of the second aspect.

According to a fourth aspect of the present invention, there is provided a portable communications device, configured for forming part an audio system further comprising:

-   -   an audio accessory; and         a cable, suitable for connecting the portable communications         device and the audio accessory, having at least one wire and         being detachable from at least one of the portable         communications device and the audio accessory,     -   the portable communications device comprising:         transmission circuitry for, at first times, transmitting on the         at least one wire first synchronisation data pattern, wherein         said first synchronisation data pattern comprises first signal         level transitions on the at least one wire, synchronised to a         master transmission clock;         receiver circuitry for, at second times, receiving from said         accessory a second synchronisation data pattern, wherein said         second synchronisation data pattern comprises second signal         level transitions; and for sampling said second signal level         transitions as received at said device; and         a controller for determining timings of said second signal level         transitions relative to the master transmission clock, and for         generating timing delay control data based on said         determination; and         wherein the transmission circuitry is configured for         transmitting to said accessory said timing delay control data.

The transmission circuitry may generate a pair of differential physical signals for transmitting on said at least one wire.

The receiver circuitry may be configured to extract data symbols from a pair of differential physical signals.

According to a fifth aspect of the present invention, there is provided a method of compensating for a round-trip transmission delay in an audio system comprising:

-   -   a portable communications device;     -   an audio accessory; and         a cable, suitable for connecting the portable communications         device and the audio accessory, having at least one wire that is         detachable from at least one of the portable communications         device and the audio accessory,     -   the method comprising, in the audio accessory:         at first times, receiving from said device a first         synchronisation data pattern, wherein said first synchronisation         data pattern comprises first signal level transitions on the         wired communications link, synchronised to a master transmission         clock;         at second times, transmitting, on the at least one wire to said         device, a second synchronisation data pattern, wherein said         second synchronisation data pattern comprises second signal         level transitions at timings that are set based on a delay value         stored in said accessory;     -   at third times, receiving timing delay control data from said         device; and     -   updating said stored delay value based on said timing delay         control data.

The method may comprise:

-   -   repeatedly iterating the steps of transmitting the second         synchronisation data pattern, receiving said timing delay         control data, and updating said stored delay value based on said         timing delay control data, until a termination condition is met.

The method may further comprise, after performing said method, entering an operational mode.

The method may comprise, in the operational mode, in each of a plurality of frames:

-   -   receiving a synchronisation data pattern from said device;     -   receiving first payload data from said device;     -   transmitting second payload data to said device; and     -   transferring control data between the device and the accessory,     -   wherein transmissions from the accessory are at timings         determined by the stored delay value, and     -   wherein the format of the frame is such that, irrespective of         whether the control data is received from the device or         transmitted to the device, there is only one pair of reversals         of a direction of data transfer in each frame.

A plurality of said frames may form a superframe, and control data in the frames of a superframe may form a plurality of control words.

According to a sixth aspect of the present invention, there is provided an audio accessory, configured for forming part of an audio system further comprising:

-   -   a portable communications device; and         a cable, suitable for connecting the portable communications         device and the audio accessory, having at least one wire and         being detachable from at least one of the portable         communications device and the audio accessory,     -   the audio accessory being adapted to compensate for a round-trip         transmission delay between the portable communications device         and the audio accessory by performing the method according to         any embodiment of the fifth aspect.

According to a seventh aspect of the present invention, there is provided an audio accessory, configured for forming part of an audio system further comprising:

-   -   a portable communications device; and         a cable, suitable for connecting the portable communications         device and the audio accessory, having at least one wire and         being detachable from at least one of the portable         communications device and the audio accessory,     -   the audio accessory comprising:         receiver circuitry for, at first times, receiving from said         device a first synchronisation data pattern, wherein said first         synchronisation data pattern comprises first signal level         transitions on the wired communications link, synchronised to a         master transmission clock;         transmission circuitry for, at second times, transmitting, on         the at least one wire to said device, a second synchronisation         data pattern, wherein said second synchronisation data pattern         comprises second signal level transitions at timings that are         set based on a delay value stored in said accessory; and     -   a controller, for receiving timing delay control data from said         device and updating said stored delay value based on said timing         delay control data.

The audio accessory may comprise a register, for storing said delay value.

The audio accessory may comprise:

-   -   a clock recovery circuit, for recovering a clock signal from         symbols received from said device; and     -   a variable delay element, for applying a controllable delay to         the recovered clock signal and generating a transmit clock for         timing symbols transmitted by said transmission circuitry,         wherein the controllable delay is determined based on the stored         delay value.

According to an eighth aspect of the present invention, there is provided a method of compensating for a round-trip transmission delay in a system comprising:

-   -   a first device;     -   a second device; and         at least one wire connecting the first device and the second         device,     -   the method comprising:         at first times, transmitting on the at least one wire, from said         first device to said second device, a first synchronisation data         pattern, wherein said first synchronisation data pattern         comprises first signal level transitions on the at least one         wire, synchronised to a master transmission clock;         at second times, transmitting, on the at least one wire, from         said second device to said first device, a second         synchronisation data pattern, wherein said second         synchronisation data pattern comprises second signal level         transitions at timings that are set based on a delay value         stored in said second device;         in said first device, determining, relative to the master         transmission clock, timings of said second signal level         transitions as received at said first device;         transmitting, from said first device to said second device,         timing delay control data based on said determination;     -   in said second device, receiving said timing delay control data;         and     -   updating said stored delay value based on said timing delay         control data.

According to a ninth aspect of the present invention, there is provided a method of operation of a half-duplex wired digital audio communications link between a master device and a slave device, the method comprising, in at least a calibration mode:

the master detecting a relative delay between transitions in data transmitted by the master and transitions in data from the slave as received by the master;

the master transmitting control data based on the relative delay to the slave device;

the slave updating a delay value stored in the slave in response to the transmitted control data; and

the slave transmitting data with a delay according to the stored delay value relative to the transitions in data as received from the master.

According to a tenth aspect of the present invention, there is provided a method of operation of a half-duplex wired digital audio communications link between a master device and a slave device, the method comprising, in at least a calibration mode:

the master detecting a relative delay between transitions in data transmitted by the master and transitions in data from the slave as received by the master;

the master transmitting control data based on the relative delay to the slave device;

the slave transmitting data with a delay, relative to the transitions in data as received from the master, the delay being based on the transmitted control data.

According to an eleventh aspect of the present invention, there is provided a half-duplex wired digital audio communications link between a master device and a slave device, comprising:

transmission circuitry in the master device for transmitting logic signals representing control data to the slave device down a wire;

receiver and detection circuitry in the master device for detecting a relative delay between transitions in said transmitted logic signals and transitions in logic signals from the slave as received by the master on the same wire and generating control data based on the relative delay; clock recovery circuitry in the slave device for recovering a receive clock from transitions in logic signals received on said wire; receiver and controller circuitry in the slave device for extracting said control data and updating a delay value stored in a storage register in the slave; and transmission circuitry in the slave device for transmitting logic signals with a delay according to the stored delay value relative to the transitions in data as received from the master.

When multiple method steps are specified, it will be appreciated that the order in which the steps are carried out may or may not be the order in which the steps are listed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: —

FIG. 1 illustrates a system in accordance with an aspect of the invention;

FIG. 2 illustrates a logical topology of the system;

FIG. 3 illustrates a frame structure;

FIGS. 4 to 6 illustrate the timing of signals between first and second devices;

FIG. 7 illustrates a command frame structure;

FIGS. 8 to 17 illustrate the format of various commands;

FIG. 18 illustrates first and second devices configured for operating as described herein;

FIG. 19 is a flow chart, illustrating a method of calibration and synchronisation;

FIG. 20 is a timing diagram, illustrating signals transmitted between the first and second devices at a first stage of the method of FIG. 19;

FIG. 21 is a timing diagram, illustrating signals transmitted between the first and second devices at a second stage of the method of FIG. 19;

FIG. 22 is a timing diagram, illustrating signals transmitted between the first and second devices at a third stage of the method of FIG. 19;

FIG. 23 is a flow chart, illustrating an aspect of the method of calibration and synchronisation;

FIG. 24 illustrates the method of FIG. 23;

FIG. 25 is a timing diagram, illustrating a signal received by a master device; and

FIG. 26 is a timing diagram, illustrating signals transmitted between the first and second devices at a fourth stage of the method of FIG. 19.

DETAILED DESCRIPTION

FIG. 1 shows an audio system, as just one example of a system using the methods described herein.

Specifically, FIG. 1 shows a host device 10, which in this example takes the form of a smartphone, but which may be any device, in particular a portable communications device. The method may be used in situations where the host device is a portable audio device, but may also be used in other situations. In this example, the host device has audio processing capability.

FIG. 1 also shows an accessory device 20, which in this example takes the form of a pair of earphones, but which may be any device, in particular any audio accessory device. In this example, the pair of earphones has two earpieces 22, 24, each of which includes a speaker for reproducing sound in response to audio signals transferred from the host device 10. Each of the earpieces 22, 24 also includes at least one microphone, for example for detecting ambient noise in the vicinity of the wearer.

Signals representing the ambient sound are then transferred from the earphones to the host device 10, which performs a noise cancellation function using an algorithm and generates anti-noise signals that it transfers to the earphones for playback. The effect of playing back the anti-noise signals is that the level of ambient noise heard by the wearer is reduced, and the wanted sounds (music, speech, or the like) that are also being transferred from the host device 10 are therefore more audible.

The accessory device 20 may be connected to the host device 10 by means of a cable 30. The cable 30 is detachable from at least one of the portable communications device and the audio accessory. In some embodiments, the cable 30 is permanently attached to the accessory device 20, and may be provided with a conventional 4-pole TRRS jack 32, to allow mechanical and electrical connection to or disconnection from the host device 10 via a matching 4-pole socket 34 provided on the host device. In other examples, the cable may be in any suitable format, for example it may have a USB Type-C connector, which similarly has four poles available for an audio interface. As a further alternative, the accessory device (or the host device) may be provided in a USB Stick dongle, with data being transferred between the dongle and the other device over a USB Type-C-compliant wired communications link.

In other cases, the accessory device 20 is permanently connected to the host device 10, and both may be parts of a larger piece of equipment. For example, the accessory device may be a microphone/speaker module provided on a widescreen television or the like, while the host device may be an audio codec circuit within the television. The methods described herein are of particular assistance when the television is large enough that transmission delays along a wire connecting the microphone/speaker and the audio codec become potentially significant.

FIG. 2 shows the logical topology of the system. Thus, FIG. 2 shows a master device 40 and a slave device 50 connected through a bus 60. The master device 40 and the slave device 50 each typically have multiple audio ports A, B, C, D for example. The method described herein allows isochronous streams of data to flow between these ports, either from master to slave (referred to herein as the “down” direction) or from slave to master (referred to herein as the “up” direction). Any port on the bus is either a source or a sink. Source ports provide the isochronous streams, while sink ports receive them. Source and sink ports can both live on either master or slave.

For illustration, in the example shown in FIG. 1, the master device 40 may be provided in the host device 10, while the slave device 50 may be provided in the accessory 20.

In this case, the source ports are devices that generate data, for example such as a microphone in one of the earpieces 22, 24, or an output of a processing device in the host device 10. The sink ports are devices that receive data, for example such as a speaker in one of the earpieces 22, 24, or an input of a processing device in the host device 10.

As examples, the ports may provide or receive oversampled audio data, such as pulse-density modulated (PDM) data, or pulse-code modulated (PCM) data, or bulk data, as examples. In the case of audio data, using oversampled data allows the data to be generated, transmitted, and processed with lower latency, and is therefore advantageous for transferring data used in noise cancellation processing where there are important time constraints on the time taken to generate the noise cancellation signals from the detected ambient noise, while PCM, being a lower sample rate but multi-bit, inherently gives rise to longer latency but is generally more efficient in terms of overall bit rate.

In operation, data is transmitted in a half-duplex fashion up and down a single wired communications link. This may be a single wire, or preferably a balanced pair of wires to allow differential transmission. A third wire may be used to provide power from the host to the accessory or to communicate information. A fourth wire may serve as ground return. Thus a standard 4-pole jack and socket arrangement may be used. Alternatively three or four (or more) wires of a standard USB or USB-On the Go connector or a multi-pole connector such as the USB Type-C connector may be used.

Data bits are encoded into symbols. In a preferred embodiment, by way of example only, a single symbol encodes a single bit, such that transmission of a data bit with a value of ‘0’ corresponds to a logic high level applied on a first wire and a logic low level applied on a second wire, whereas transmission of a data bit with a value of ‘1’ corresponds to a logic low level applied on a first wire and a logic high level being applied on a second wire. The described invention can be applied using various encoding methods, irrespective of the number of wires involved to encode the symbols, and of the number of data bits that are encoded in a single symbol.

FIG. 3 shows the format in which data is transferred between the master device and the slave device. The interfaces from the master host device to the slave accessory are designed and configured in use such that data is transmitted in the format of a superframe, which in the illustrated example comprises 128 frames, numbered from 0 to 127 in the first column of FIG. 3. Each frame has the same configurable number (N_Symbol) of symbol slots.

The number N_Symbol may for example be configurable to be in the range of 8 to 20. For example, N_Symbol may have a default value of 12, which can be varied if required.

During each symbol slot, one and only one of the interfaces on the bus (i.e. the master or the slave) can transmit a symbol on the bus. If the two interfaces simultaneously attempted to drive a data symbol on the bus, conflicting drive currents would prevent correct transfer of data symbols between the interfaces, even if the time during which the two interfaces drive simultaneously only comprises a fraction of a symbol slot period. Therefore the frame and superframe structure enables a strict orchestration in which each specific symbol slot is allocated for either the master or the slave to be transmitting while the other is receiving.

In addition, correct transfer of data symbols requires that a first interface does not transmit during any next symbol slot until any symbol as transmitted in the preceding symbol slot by a second interface on the bus has been completely transferred, including the transmission delay over the cable associated with the transfer.

In this example, each frame contains only two synchronisation (sync) symbols, which are transmitted from the master, and no sync symbols are transmitted from the slave. In this example, the two synchronisation (sync) symbols are the first two symbols of each frame.

In this example a predetermined signal pattern is transmitted during the sync symbol slots, such that the slave device can expect this signal pattern in the same position in each frame and can use the transition in the received signal level between the two sync symbol slots to establish and/or maintain synchronisation of its clock. In this example the first and second symbol slots encode a logic ‘0’ and ‘1’ respectively.

Each frame also contains a number of symbol slots dedicated for use for transmitting control data. In the illustrated example, only a single symbol slot for control data per frame is provided, as this will be adequate for many purposes. Control data may be transmitted from the master to the slave in some frames, and from the slave to the master in other frames. Where there are multiple symbol slots allocated for control data in each frame, it is possible to allocate one or more of these symbol slots for transmitting control data from the master to the slave, and one or more of these symbol slots for transmitting control data from the slave to the master, in at least some frames.

Each frame may be configured in use to allow transmission of a number of symbols (Nd) of payload data down from master to slave and a number of symbols (Nu) of payload data up from slave to master in a half-duplex fashion. These numbers may be the same for all frames of the superframe.

To avoid overhead or dead time when changing transmission direction, the order of the sync symbols, control symbols and signal (i.e. payload) data up and down is constrained so that only one pair of reversals of transmission direction occurs in each frame. A single symbol slot may thus be adequate to accommodate a maximum transmission delay between master and slave and back again from slave to master. FIG. 3 shows this single symbol slot as “unused”. The position of this unused symbol slot in the frame is explained in more detail below.

Thus the maximum total number of up and down signal data symbols may be N_symbol less the total number of sync symbols, control symbols, and the one further symbol to allow for transmission delay. In the case of two sync symbols, and one control symbol, this allows the transmission of up to (N_symbol−4) signal data symbols per frame. For N_symbol of, say, 20 this gives a symbol slot utilisation factor of 80%. In other words, the total number of symbol slots usable for sync symbols, control symbols, data up and data down is only one less than N_symbol.

The common length (N_symbol) of the frames in a superframe and the allocation of symbols in the various frames of the superframe may be programmed on the fly in use. The frame rate may be predetermined, and the symbol rate increased to accommodate larger numbers of symbols per frame, to preserve the latency, rather than keeping the symbol rate constant and reducing the frame rate.

The allocation of the data symbols may be different from frame to frame in a superframe. This flexibility advantageously allows bitstream oversampled data to pass through with minimal delay, while allowing multi-bit PCM words to be packed efficiently and conveniently and allowing minimisation of any transmission latency. The bidirectional control channel may also be advantageously reconfigured on-the-fly, to accommodate different scenarios requiring more or less control data to be passed up or down the link.

FIG. 3 shows how different data streams may be transmitted on the bus at the same time. Thus, FIG. 3 shows:

a first PCM data stream (PCMA) being transmitted from the slave to the master in two symbol slots of each even-numbered frame;

a second PCM data stream (PCMB) being transmitted from the slave to the master in three symbol slots of each odd-numbered frame;

a third PCM data stream (PCMD) being transmitted from the master to the slave in three symbol slots of the first two frames of each block of four consecutive frames;

a fourth PCM data stream (PCME) being transmitted from the master to the slave in one symbol slot of the last two frames of each block of four consecutive frames;

a first PDM data stream (PDMA) being transmitted from the slave to the master in one symbol slot of each frame; and

a second PDM data stream (PDMF) being transmitted from the master to the slave in one symbol slot of each frame.

FIG. 3 also shows how the control symbols are transmitted from the master to the slave in some frames (for example frames 0, 1 and 2) and from the slave to the master in other frames (for example frames 3 and 4).

FIG. 4 shows the effect of the transmission delays. Specifically, in FIG. 4, the upper graph shows the frame format with time as observed on the master side, and the bottom graph shows the perception of time on the slave side. In each graph, symbols above the respective central horizontal line represent the symbols driven by the master, while symbols below the respective central horizontal line represent the symbols driven by the slave. The frame period is defined in this diagram as t_(frame), while the symbol period is defined as t_(symbol).

Thus, the first two symbols are the two sync symbols, Sync0 and Sync1, two consecutive symbol slots at fixed opposite signal levels, enabling the slave device to recover the symbol timing.

As shown in FIG. 4, the two sync symbols are received by the slave device slightly later than the times at which they were transmitted by the master device.

The sync symbols are followed by the control (ctrl) symbol. Together the 128 control symbols of a single superframe, one taken from each frame, constitute the control frame, as described in more detail below. Each symbol in the control frame may flow in either direction. The direction in which each symbol of control data flows is defined by prior data bits in the control frame and is governed by the master.

FIG. 4 illustrates two alternative possibilities, which cannot both take place in the same frame. Thus, FIG. 4 shows a control symbol being transmitted by the master at 70 and received by the slave at 72 after the transmission delay. FIG. 4 also shows the alternative possibility, with a control symbol being transmitted by the slave at 74 and received by the master 76 after the transmission delay.

The following N_(Up) symbol slots carry audio data from the slave. There is an empty slot in which neither the master nor the slave transmit any data. Note that on the master side a significant fraction of the last data symbol of the up data from the slave will be coming in during this empty slot. Data bits during these N_(UP) symbol slots in the superframe can be encoded and grouped in a series of consecutive symbols to convey audio streams between source ports on the slave and sink ports on the master.

The following N_(Down) symbol slots carry audio data from the master to the slave. Similarly data bits during this phase can be encoded and grouped in a series of consecutive symbols to convey audio streams between source ports on the master and sink ports on the slave.

The number of symbol slots allocated for data is determined by the total number of symbol slots per frame. That is, (N_(UP)+N_(Down))≦(N_Symbol−4).

FIG. 5 illustrates one example of the timing in more detail.

As before, each frame has two sync symbols, transmitted from the master to the slave, then a control symbol which may be transmitted in either direction, then a series of data symbols D₁, D₂, . . . , D_(Nu) transmitted from the slave to the master, and a series of data symbols D₁, D₂, . . . , D_(Nd) transmitted from the master to the slave. A transmission delay applies to transmissions in both directions. Symbols transmitted from the slave to the master are illustrated with slightly lower heights than symbols transmitted from the master to the slave, but this is simply for illustrative purposes and has no physical meaning.

Specifically, FIG. 5 illustrates the case in which the bidirectional control slot is allocated for transmission of control data from the master to the slave.

In this case, the slave receives the control symbol, and then transmits the first data up symbol after a delay indicated by the reference numeral 90. Because of the transmission delays in both directions and the delay 90 within the slave, there is a delay indicated by the reference numeral 92 between the time at which the master transmits the control symbol and the time at which the master receives the first data up symbol. During this delay 92, the master device enters a high impedance state Z, during which there is no observable effect at the master device of either device driving the wire. Similarly, during the delay 90, the slave device enters a high impedance state Z, during which there is no observable effect at the slave device of either device driving the wire.

Data from the master to the slave is transmitted synchronous to a master symbol transmission clock (not illustrated) corresponding to a symbol period. Thus when the direction of data transfer is reversed again, the first data symbol down is not transmitted until a whole number of symbol periods since the previous symbol transmitted down (in this case the control symbol sent just prior to delay 92). Thus there is a further delay 94 between when the master receives the last data up symbol until it transmits the first data down symbol. Because of the transmission delays in both directions and this additional delay 94, there is a delay indicated by the reference numeral 96 between the time at which the slave transmits the last data up symbol and the time at which the slave receives the first data down symbol. During the delay 94, the master device enters a high impedance state Z, during which there is no observable effect at the master device of either device driving the wire. Similarly, during the delay 96, the slave device enters a high impedance state Z, during which there is no observable effect at the slave device of either device driving the wire.

Thus, there is a delay at each device when the direction of data flow is changed. The two delays 92, 94 seen at the master device have the same total length as the two delays 90, 96 seen at the slave device. In this example, these two delays, in each case, total one symbol period t_(symbol). Because there are only two such reversals of the data transfer direction in each frame, the total time associated with the transmission delay is smaller than it would be with more direction changes.

FIG. 6 illustrates another example of the timing, with the same frame structure as in FIG. 5, but in the case in which the bidirectional control slot is allocated for transmission of control data from the slave to the master.

In this case, the slave receives the second sync symbol, and then transmits the control data symbol after a delay indicated by the reference numeral 100. Because of the transmission delays in both directions and the delay 100 internal to the slave, there is a delay indicated by the reference numeral 102 between the time at which the master transmits the second sync symbol and the time at which the master receives the control symbol.

There is in this case no need for a delay between the transmission of the control symbol and the first data up symbol. There is a further delay only when the direction of data transfer is reversed, That is, when the master receives the last data up symbol, it transmits the first data down symbol after a delay indicated by the reference numeral 104 due to waiting for the next edge of the master symbol transmission clock. Because of the transmission delays in both directions and this delay 104, there is a delay indicated by the reference numeral 106 between the time at which the slave transmits the last data up symbol and the time at which the slave receives the first data down symbol.

Again, there is a delay at each device when the direction of data flow is changed, and there are only two such direction changes in each frame, even though the control symbol is in the opposite direction to that shown in FIG. 5. The two delays 102, 104 seen at the master device have the same total length as the two delays 100, 106 seen at the slave device. Again, these two delays, in each case, total one symbol period t_(symbol). As before, during the delays 102, 104, the master device enters a high impedance state Z, during which there is no observable effect at the master device of either device driving the wire. Similarly, during the delays 100, 106, the slave device enters a high impedance state Z, during which there is no observable effect at the slave device of either device driving the wire.

This ensures that a maximally large portion of the frame period can be used effectively for transferring data, inherently optimizing the number of data symbols that can be transferred per unit of time, given the constraints for latency and the required overhead to synchronize two devices on either side of the interface. As a consequence of the ordering of the symbol slots as described, the direction of the data in the control symbol has no impact on the maximal number of data symbols that can be transferred per unit of time.

FIG. 7 shows the structure of a control frame, as discussed above. In this example, the control frame consists of one symbol in each frame of a superframe, so the control frame comprises 128 symbols in total, where each symbol encodes a single bit

As mentioned above, data in the control frame can flow in either direction. The direction in which the data flows is defined by a set of rules, which ensure that both master and slave know the direction during each symbol slot that is part of the control frame.

The following description of the control frame talks about bit slots, assuming that any symbol slot conveys one bit, though the described mechanism can be applied for any encoding scheme (i.e. 2 or more data bits per symbol) as long as data symbols only encode data bits flowing in a single direction.

In this illustrated example:

bits 0-15 of the control frame consist of a Master Super Frame Cyclic Redundancy Check (CRC), MSFCRC, holding the CRC over the data sent by the master in the previous Super Frame;

bits 16-21 of the control frame indicate the status of the Master;

bits 22-37 of the control frame consist of a Slave Super Frame CRC (SSFCRC) containing the CRC over the data sent by the slave in the previous Super Frame; and

bits 38-55, 56-73, 74-91, 92-109 and 110-127 contain a sequence of five commands, CMD0, CMD1, CMD2, CMD3 and CMD4, each consisting of eighteen bits, and used to perform control data transfer between the master and the slave.

As mentioned above, bits 16-21 of the control frame indicate the status of the Master. More specifically, in one example, the 6-bit Master Status holds the following Master Status bits:

Section Bit Slots Description VERSION 5 . . . 4 This has the purpose to provide forward compatibility. Current version: 0b00. SYNC_ST 3 . . . 2 Sync Status: NO: 0b00 LINK: 0b01 UP: 0b10 FULL: 0b11 Thus, the master can indicate whether it has yet achieved partial or full synchronisation with the slave device LAST_BIT 1 When LAST BIT is set to 1 the slave will send out a SSFCRC in the next Frame. IGNORE 0 When this bit is set to 1 a slave device will ignore the remainder of the control frame, and will not send out a SSFCRC

In standard implementations the CRC is padded with ‘0’ bits before the actual transmission of the CRC code itself. As a consequence a remainder of the division by the CRC polynomial of a to be protected received bit sequence in concatenation with the received CRC code itself results in a remainder of all ‘0’s when no bit errors have been introduced during the transfer of data.

By padding the CRC with a modified padding sequence this remainder will have a fixed value other than all ‘0’s, independent of the data sequence to be protected.

By padding the CRC with a specific padding sequence the remainder can be guaranteed to be all ‘1’s in the absence of transmission errors.

A characteristic of CRC calculation is that when an arbitrary number of errors is introduced in the transferred data sequence the calculated remainder on the receiving end can obtain each value with the same likelihood. As a consequence the impact on the robustness on the CRC calculation by allowing for a single modified padding sequence to provide a binary messaging mechanism such as associated with the transfer of interrupts only increases the probability of not detecting an erroneously received data sequence by a factor of 2. Such loss of robustness is in forthcoming scenarios acceptable.

The control frame also allows the slave, by applying the above described method of using a modified padding sequence for the calculation of the SSFCRC in the symbol slots allocated for transmitting the SSFCRC, to send an interrupt request to the master.

As mentioned above, each control frame allows the transmission of five commands, each consisting of eighteen bits. These eighteen bits consist in this example of two bits to encode four different possible commands.

In this example, the four possible commands are:

Command Operand Operand From ADDR 16 bit address MA RD 16 bit data word to be read by master from SL slave WR 16 bit data word to be written by master to MA slave CRC 16 bit CRC over previous transfer SL/MA

Each command is followed by a 16-bit operand or return value. This ensures that all commands fall in an 18 bit grid. The fixed grid ensures that the slaves can always recover their position within a sequence of commands, minimizing the risk of further escalation of errors when a single bit in the command is detected incorrectly by a slave device.

Read and write transfers are formed by a concatenation of multiple commands plus operands or return values. The rules defining how specific sequences of basic commands result in various more complex transfers ignore the control frame boundary. So, for example, a command occupying two of the 18-bit slots may use two consecutive slots (for example the CMD0 and CMD1 slots) in one superframe, but could just as well use the CMD4 slot in one frame and the CMD0 slot of the subsequent frame.

Thus, commands are all issued by the master device. The default command, when the master is not actively transferring, is the CRC command. Each transfer sequence starts with an ADDR command. A second ADDR command may be used to extend a 16-bit partial address to a 32-bit full address.

One or two ADDR commands are then followed by one or more RD or WR commands. Subsequent RD or WR commands address subsequent registers (thus assuming auto incrementing). The end of a RD or WR transfer is marked by a CRC or subsequent ADDR command.

The first CRC after a WR comes from the master to protect the content of the transfer, including the address. The second CRC after a WR then comes back from the slave, to reliably confirm the reception of the data. The first CRC after a RD comes from the slave, protecting the content of the transfer.

An integrity buffer on the slave enables checking and purging/approving of write transfers before such write transfers are passed on, for instance into registers inside the slave device. The integrity buffer is only used when the slave is in integrity mode. Otherwise, register writes may be directly fed into the registers.

FIG. 8 shows the format of a command for a basic 16 bit data, basic 16 bit address write transfer, consisting of a single ADDR command followed by a WR command. The 16 bit address allows for the addressing of a subset of the devices and ports that can be addressed by a 32 bit address.

FIG. 9 shows the format of a command for a 16 bit data, full 32 bit address write transfer, consisting of two ADDR commands followed by a WR command:

FIG. 10 shows the format of a command for a 4×16 bit block, basic 16 bit address write transfer, consisting of an ADDR command followed by four WR commands. This is just an illustrative example, as any number of WR commands can follow a single ADDR command. Subsequent bytes words are stored in subsequent address locations. This auto increment function only applies to the RA section of the address, not the DA or PA.

FIG. 11 shows the format of a command for a basic 16 bit data, basic 16-bits address read transfer, consisting of an ADDR command followed by a RD command, with the data, data0, in this case being transferred from the slave to the master.

Similarly, as shown in FIG. 12, using two ADDR commands allows full 32 bit addressing.

Also, as shown in FIG. 13, multiple consecutive RD commands form a block read command.

Block writing and reading can be combined with Full 32 bit addressing, as well as with basic addressing.

The command scheme also allows for the use of robust reading and writing. A robust write transfer is identical to a non-robust write transfer, with the difference that a robust transfer is concluded by a CRC Command and a CRC from the master. A second CRC command can be given, to which the slave will respond through a CRC to acknowledge the reception of the previous robust write transfer

A robust read transfer is identical to a non-robust read transfer, except for the addition of the CRC command and the following 16-bit CRC from the slave.

The robust write transfers make assume the presence of an integrity buffer. During the transfer (read or write) retrieved data is written into the integrity buffer. Data is only retrieved from the integrity buffer once the integrity of the transfer has been confirmed by means of the CRC concluding the transfer. Only then it will find its way into the registers of the slave in case of a write transfer, or be accepted by the master in case of a read transfer.

FIG. 14 shows the command sequence for a robust block read, using basic addressing, where data0, data1 and crc16 are sent from the slave to the master, and the a horizontal line with circles at each end indicates which data bits (CMD+OPERAND/RESPONSE) are covered by the respective CRC.

FIG. 15 shows the command sequence for a robust 1-word write, with full addressing, and therefore having two ADDR commands, one WR command, and one CRC command.

FIG. 16 shows the command sequence for a robust 1-word write, with basic addressing, and with acknowledge and therefore having one ADDR command, one WR command, and two CRC commands, with data0, and the first crc16 being sent from the master to the slave and the second crc16 being sent from the slave to the master.

FIG. 17 shows the command sequence for a robust long block write, with basic addressing, and therefore having one ADDR command, four WR commands, and two CRC commands, with data0, data1, the first crc16, data2, data3 and the second crc16 being sent in that sequence from the master to the slave.

FIG. 18 shows an embodiment of a system as described above, showing a master device 120 and a slave device 122, connected by a bus 124. The bus 124 is illustrated as a single wire, though physically the connection may comprise a single wire for signal transmission or preferably a balanced pair of wires to allow differential signal transmission, as well as possibly other wires for power and ground connections or other signals.

In this embodiment, the timing on the master side is set by a master clock BCKM_R, at the symbol rate, and the frame clock FCKM is obtained by dividing that clock in a divider 126 by the number of symbols per frame, N_symbol. (In other embodiments FCKM may be supplied as a master clock and BCKM_R generated therefrom by a clock multiplier, or both clocks may be divided down from some higher-frequency master clock)

The phase of the BCKM_R clock corresponds to the timing at which the master device 120 expects to receive data symbols from the slave device 122. Thus, the BCKM_R clock signal is supplied through an inverter 128 to a receive block 130, which receives data DupR transmitted from the slave. This data contains traffic data, i.e. payload data, and control data, and so the received signal is passed to a demultiplexer 132, which supplies the traffic data on an output 134, and supplies the control data to a controller 136.

When the master side is transmitting data, control data from the controller 136 is multiplexed in a multiplexer 138 with the traffic data to be transferred, and the resulting combined data Ddn is supplied to an output register 140. The BCKM_R clock is passed through a delay block 142 to generate clock pulses BCKM_T whose phase corresponds to the timing at which the master device 120 intends to transmit data symbols. The BCKM_T clock signal is then used to latch the output data from the output flip-flop 140 to a buffer 144. The buffer 144 is controlled by a sequencing block 146, which, with knowledge of which symbol slots in a frame are allocated for transmission from the master to the slave, enables data to be passed through the buffer 144 during the correct time slots.

When data is received at the slave side, the pulses are passed to a clock recovery circuit 160. As mentioned previously, the presence of the sync pattern once in each frame allows the synchronisation of the slave to the master to be maintained. The clock recovery circuit 160 may for example include a phase-locked loop, with a voltage controlled oscillator generating a clock signal BCKS_R at the symbol rate. The recovered clock BCKS_R, corresponding to the timing at which the slave device 122 expects to receive data symbols from the master device 120, is passed to a divider 182 to divide the frequency of BCKS_R by N_symbol, the number of symbols per frame (say 12), to obtain a slave side frame clock FCKS with edges synchronous with the transition in the one sync pattern per frame.

Thus, the recovered clock BCKS_R is supplied through an inverter 162 to a receive block 164, which receives data DdnR transmitted from the master. This data contains traffic data and control data, and so the received signal is passed to a demultiplexer 166, which supplies the traffic data on an output 168, and supplies the control data to a controller 170.

When the slave side is transmitting data, control data from the controller 170 is multiplexed in a multiplexer 172 with the traffic data to be transferred, and the resulting data Dup is supplied to an output register 174. The BCKS_R clock is passed through a controllable delay block 176 to generate clock pulses BCKS_T whose phase corresponds to the timing at which the slave device 122 intends to transmit data symbols. The BCKS_T clock signal is then used to latch the output data from the output register 174 to a buffer 178. The buffer 178 is controlled by a sequencing block 180, which, with knowledge of which symbol slots in a frame are allocated for transmission from the slave to the master, enables data to be passed through the buffer 178 during the correct time slots.

As described in more detail below, the delay applied by the controllable delay block 176 is determined by a delay value that is stored in a register 184. The delay value stored in the register 184 can be updated based on timing control data that forms part of the control data transmitted by the master device 120 to the slave device 122 and extracted by the controller 170 from the received signal DdnR.

The controllable delay block 176 can for example take the form of a delay line, comprising multiple inverters, each delaying the signal by a time of the order of nanoseconds, such that the total delay applied by the controllable delay block 176 depends on the number of such elements that is switched into the signal path.

Thus, the operational mode of the system is described above. Before entering the operational mode, there is a calibration and synchronisation phase, which establishes the connection between the master and the slave.

In this embodiment, the calibration and synchronisation phase contains five sub-phases, although it will be appreciated that any of these sub-phases can be performed separately from the others, or the sub-phases may be performed in a different order.

FIG. 19 is a flow chart, illustrating the calibration and synchronisation phase of one particular embodiment.

Specifically, FIG. 19 shows an initial step, which takes place on system start-up, power-up, or reset, or at a later time, when it is detected that an accessory jack has been inserted into the socket of the master device, or at a time when the system is already powered up with a jack inserted and an audio application is first started. At this stage, there is no synchronisation between the master portable communications device and the audio accessory. In step 190, the master device sends symbols to the slave device, allowing the slave to establish initial synchronisation.

In step 192, down synchronisation takes place, in which the slave sends symbols to the master device, allowing the master to enter a synchronised state.

In step 194, the slave re-acquires frame lock.

In step 196, the slave sends synchronisation symbols to the master, which uses the received synchronisation symbols to determine the timings of the received symbols relative to the timings of its own transmissions. The master device then sends timing delay control data based on that determination, in an iterative procedure until the timing reaches a desired value. The slave device then stores a delay value based on that timing delay control data.

In step 198, the master and the slave devices reach full synchronisation.

FIG. 20 is a timing diagram, showing in more detail the symbols sent between the master device and the slave device in step 190 of the process shown in FIG. 19.

When it is powered up, the master device is generating a master transmission clock signal BCKM_T, which it uses for timing its transmissions. Specifically, at this time, the master device sends out a basic pattern that can be used by the slave device to establish timing information. Thus, the master device sends a synchronisation data pattern that consists of a 0 (shown as 0↓ in FIG. 20) and a 1 (shown as 1↓ in FIG. 20) in consecutive bit slots of each frame.

In the first 16 frames of each repeating superframe of 128 frames, in the bit slot following the synchronisation data pattern, the master device sends a control bit (shown as C↓ in FIG. 20). In the period of each frame following the control bit, each device enters a high impedance state Z, during which there is no observable effect at that device of either device driving the wire.

As shown in FIG. 20, the symbols of the synchronisation data pattern transmitted by the master device are received by the slave device after a transmission delay of t_(pD). The slave device contains circuitry (for example the clock recovery circuitry 160 shown in FIG. 18) for recovering a local slave reception clock BCKS_R based on the timing of the 0 to the 1 transition that occurs in each frame.

Once the slave reception clock BCKS_R has been established, a clock edge that is one and a half symbol periods later than the 0 to the 1 transition in the synchronisation data pattern is used to sample the control bits in the frames in which they occur. The presence of these control bits in the first 16 frames of each superframe, as described above, is then used by the slave device to identify the start of the superframe.

Once the slave device recognises this, it enters a down synchronisation state.

FIG. 21 is a timing diagram, showing in more detail the symbols sent between the master device and the slave device in step 192 of the process shown in FIG. 19.

Specifically, FIG. 21 shows the situation in a part of the control frame that is outside the first 16 frames of the superframe, as this latter period is being used for the master device to send a control word as described above.

The master device is continuing to send out the basic synchronisation data pattern consisting of a 0 (shown as 0↓ in FIG. 21) and a 1 (shown as 1↓ in FIG. 21) in consecutive bit slots of each frame.

As mentioned with reference to FIG. 20, the symbols of the synchronisation data pattern transmitted by the master device are received by the slave device after a transmission delay of t_(pD).

Immediately after receiving the symbols of the synchronisation data pattern, the slave device sends a control symbol (shown as C↑ in FIG. 21), which is double the normal length, that is, it occupies two clock periods of the slave reception clock BCKS_R. This double length symbol allows the master device to sample the received symbol at a point that is well separated from a bit transition, even though the timing of the received signal will be unknown to the master device at that time.

The control symbols C↑ transmitted by the slave device over multiple frames form a control sequence.

FIG. 21 shows the double length symbol C↑ transmitted by the slave device to be received by the master device after a transmission delay of t_(pU), so that there is a total gap, shown by the reference numeral 202, equal to t_(pD) plus t_(pU), between the time at which the master device completes sending the synchronisation data pattern and starts receiving the control symbols C↑ transmitted by the slave device. During the delay 202, the master device enters a high impedance state Z, during which there is no observable effect at the master device of either device driving the wire.

The control symbols transmitted by the slave device contain some information that confirms that the slave device is a valid slave, operating in accordance with the expected protocol.

On receipt of a valid control sequence, the master device enters the down synchronised state.

One this state has been established, the master device is able to send control data to the slave device in order to program the desired value of N_Symbol into registers in the slave. Once the slave has confirmed (by sending suitable control bits) that this control data has been received, the master instructs the slave to start using the programmed value of N_Symbol instead of its previous default value (for example, 16).

In step 194 of the process shown in FIG. 19, once the slave device has stored the desired value of N_Symbol, it is able to re-acquire the suitable frame lock, to achieve link synchronisation.

Once the slave confirms this, and the master has verified it, the master device enters the up synchronised state.

FIG. 22 is a timing diagram, showing in more detail the symbols sent between the master device and the slave device in step 196 of the process shown in FIG. 19.

The master communicates to the slave device that it is in the up synchronisation state. In this stage, as shown in FIG. 22, the master device continues to send out the basic synchronisation data pattern consisting of a 0 (shown as 0↓ in FIG. 22) and a 1 (shown as 1↓ in FIG. 22) in consecutive bit slots of each frame.

As mentioned with reference to FIG. 20, the symbols of the synchronisation data pattern transmitted by the master device, and hence the transitions between the symbols of the synchronisation data pattern, are received by the slave device after a transmission delay of t_(pD).

The slave device then starts sending out its own synchronisation data pattern comprising two symbols of opposite polarity. As shown in FIG. 22, the synchronisation data pattern transmitted by the slave device comprises a 1 (shown as 1↑ in FIG. 22) followed by 0 (shown as 0↑ in FIG. 22). (The synchronisation data pattern transmitted by the slave device may alternatively comprise a 0 followed by a 1 with appropriate adjustments to following logic.)

As shown in FIG. 22, the slave device does not send its synchronisation data pattern immediately after receiving the symbols of the synchronisation data pattern from the master device, as it did with the control symbol in FIG. 21, but rather it inserts an extra delay t_(ds) between the edges of the synchronisation data pattern it receives and the edges of the synchronisation data pattern it transmits. This extra delay t_(ds) may for example initially be set by storing in the register 184 (shown in FIG. 18) an initial delay value of half of one symbol period.

FIG. 22 shows the synchronisation data pattern transmitted by the slave device to be received by the master device after a transmission delay of t_(pU), so that there is a total gap, shown by the reference numeral 208 and equal to (t_(pD)+t_(ds)+t_(pU)), between the time at which the master device completes sending the synchronisation data pattern and the time at which it starts receiving the synchronisation data pattern transmitted by the slave device.

FIG. 23 is a flow chart, showing how the master device uses the synchronisation data pattern that it receives from the slave device.

In step 220, the initial delay value t_(ds) is set, and stored in the register 184 in the slave device.

In step 222, the master device receives the synchronisation data pattern in one frame. In one embodiment, the synchronisation data pattern is transmitted by the slave device for 16 consecutive frames.

In step 224, the master device samples the received symbols of the received synchronisation data pattern. Specifically, the master device establishes a master reception clock signal BCKM_R, which is offset from the master transmission clock signal BCKM_T by a time difference that is close to one half of one symbol period. In normal operation mode, when the calibration and synchronisation is complete, edges of this clock are used to sample signals received from the slave device, and so it is advantageous to do this sampling at a point that is far from the symbol transition point (i.e. in the middle of the received pulses). In step 224, the opposite edges of the master reception clock signal BCKM_R are used to sample the received synchronisation data pattern. The intention here is to sample the received synchronisation data pattern at time points that are as close as possible to the symbol transitions. Moreover, it is preferable for the symbol transitions of the signals received by the master device to be close to the symbol transitions of the signals transmitted by the master device, but with a sufficient margin t_(m) to ensure that a symbol received from the slave is completed before the start of transmission of a symbol from the master.

In step 226, it is determined whether 16 consecutive frames of the synchronisation data pattern have been received. If not, the process returns to step 222. When 16 consecutive frames of the synchronisation data pattern have been received, the process passes to step 228, in which the sample values obtained during the 16 frames are examined.

As mentioned above, the delay value set in the slave device, namely t_(ds), may for example initially be set to a value of half of one symbol period. If the transmission delays (t_(pD) and t_(pU)) total an additional delay of one half of one symbol period delay, the total round-trip delay t_(dRT)=t_(pD)+t_(pU)+t_(ds)=one bit period.

If this could be achieved with perfect reliability in the operational mode, then the master device would start transmission of its data symbols immediately after completing receiving data symbols from the slave device. However, various factors mean that this cannot be guaranteed. For example, the effects of clock jitter and noise means that the timings of the symbol edges will fluctuate slightly. In addition, the timings may drift due to changes in temperature, physical deformation, or the like. Therefore, it may be preferable for there to be a small margin between the master device completing receiving data symbols from the slave device and starting transmission of its data symbols.

This process therefore aims to control the delay value set in the slave device, namely t_(ds), so that the total round-trip delay (t_(dRT)=t_(pD)+t_(pU)+t_(ds)) plus this margin (t_(m)) equals one bit period.

Therefore, in step 224, the master device samples the received synchronisation data pattern at a time point that is t_(m) before the point at which the master would start to transmit symbols, as governed by the master transmission clock.

If the total round trip delay t_(dRT) is smaller than anticipated, then the master will sample late, that is, after the transition point in the slave synchronisation data pattern. Since this transition is from 1 to 0, the sample value obtained by the master will probably be a 0. If the total round trip delay t_(dRT) is larger than anticipated, then the master will sample early, and the sample value obtained by the master will probably be a 1.

If the total round trip delay t_(dRT) is exactly as anticipated, then the master will sample exactly on the transition point, and the sample value is equally likely to be a 0 or a 1.

In more detail, the transition of the physical signal is likely to have a non-zero fall time. The interpretation of whether the physical signal represents a logic 1 or 0 may involve a comparison at the sampling time of the signal level against a nominal logic threshold value. When optimally aligned the value of the signal level at the sampling point would be expected to be exactly equal to the nominal logic threshold value. Noise and similar effects would result in interpretation of a logic 1 or 0 at each sample point, each being equally likely. Thus the sampled logic level is equally likely to be a 0 or a 1, as interpreted by comparison of the sampled physical signal level relative to the expected level.

In step 228, the master device considers the sample values obtained from sampling the slave synchronisation data patterns in the block of 16 frames, and compares the numbers of 1's and 0's. If the majority of sample values are 0's it is determined that the master was sampling late; if the majority of sample values are 1's it is determined that the master was sampling early; and, if the proportions of 1's and 0's are close to 50% each, it is determined that the sampling point was approximately as intended.

In step 230, it is determined whether the error in the sampling point has changed polarity since the previous iteration, that is whether the master has changed from sampling early to sampling late, or vice versa. If not, that is, if the polarity of the error is the same as in the previous iteration, the process passes to step 232.

In step 232, the master sends updated delay value information to the slave device. This delay value information causes the slave device to alter the extra delay value t_(ds) by a predetermined step size.

If it is determined in step 230 that the error in the sampling point has changed polarity since the previous iteration, that is the master has changed from sampling early to sampling late, or vice versa, the process passes to step 234. In step 234, the predetermined step size mentioned above is modified from its previous value. For example, in one embodiment, the step size is halved in step 234. The process passes to step 232 as before.

The process then passes to step 236, in which it is determined if a termination condition has been met. If not, the process returns to step 222 but, if the termination condition has been met, the process passes to step 238, and ends. Thus, the process iterates until the termination condition is met. For example, the termination condition may require that the delay value has converged to a value of t_(ds) which is judged satisfactory, or may set a predetermined maximum number of iterations of the process, or may consider whether a predetermined minimum value of the step size has been reached by repeated operation of step 234.

FIG. 24 illustrates one operation of the method, by showing the delay value that is applied in different iterations of the process, with the optimum value being the delay value that causes the master to sample exactly at the transition point in the slave synchronisation data pattern and that (due to the effects of jitter and noise) is equally likely to produce a majority of sample values that are 0's as it is to produce a majority of sample values that are 1's.

Thus, in a first iteration A, the delay value is found to be too small, and so timing control data is sent to the slave, causing the delay to be increased by a step S. In iteration B, the delay value is again found to be too small, and so timing control data is again sent to the slave, causing the delay to be increased by a further step S.

As shown, in the iteration C, the delay value is found to be slightly too large, and so timing control data is sent to the slave. As the polarity of the difference has changed, the step size is halved, and so the new timing control data causes the delay to be reduced by a step S/2.

In the next iteration D, the delay value is again found to be too small. As the polarity of the difference has changed again, the step size is halved again, and so the new timing control data causes the delay to be increased by a step S/4. In iteration E, the delay value is again found to be too small, and so timing control data is again sent to the slave, causing the delay to be increased by a further step S/4.

As a result, in the iteration F, the delay value is found to be slightly too large. As the polarity of the difference has changed again, the step size is again halved, and so the new timing control data causes the delay to be reduced by a step S/8.

Thus, the process continues, and it can be seen that the delay value converges towards the optimum value.

It can also be seen that the effect of halving the step size only in the event of a polarity change helps with the convergence. For example, in this illustration, the delay value was found to be slightly too large in the iteration C. However, if, instead, the delay value had been found to be slightly too small then, as shown by the dotted line, the timing control data sent to the slave would have caused the delay to be increased by a further step S. Then, the delay value would have been found in step D to be too large, and so timing control data sent to the slave would have caused the delay to be reduced by a step S/2 (with the step size having been halved). Next, the delay value would again have been found in step E to be too large, and so timing control data sent to the slave would have caused the delay to be reduced by a further step S/2, thus returning to the same point in step F as was actually found to be the case. Thus, the algorithm is robust against discrepancies caused by jitter, noise, or the like.

This method can be performed using the control words that are available, as described above. Thus, FIG. 7 shows the 128 bits forming one control frame, with one control bit being present in each frame of the superframe. As shown in FIG. 7, the control frame includes five control words, that can be used to transmit control data from the master to the slave, or from the slave to the master.

In one embodiment, the method described here is iterated once in each superframe, as shown in FIG. 24. Each iteration requires four control words out of the five available in each superframe. (As described below, the superframe rate may, for example, be 12 kHz.)

For example, the first four control words CMD0-CMD3 in each superframe can be allocated as follows:

CMD0: Master transmits slave address to write updated delay into

CMD1: Master transmits Updated delay value.

CMD2: Slave transmits Dummy address Zero

CMD3: Slave transmits test pattern with updated delay, to be read by master.

In the physical embodiment illustrated in FIG. 18, these control words are received as a physical signal by the receive block 164, where the signal is sampled at an edge of an appropriate version of the receive clock BCKS_R to provide a data stream DdnR, from which the controller 170 extracts the control words CMD0 and CMD1, and writes a delay value to the register 184. This stored delay value is then used by the delay block 176 to provide a suitable clock with which to transmit the control words CMD2 and CMD3 in control frame slots defined in the sequencer 180 as physical signals driven by the buffer 178.

On arrival at the master device, the physical signal is sampled by the receive block or sampling circuit 130 at an edge of a version of local clock BCKM_R to generate the data stream DupR. The late or early arrival of the physical signal may be determined or deduced by the controller 136, for example from the relative numbers of 0's and 1's in some period, from which the controller 136 may generate an updated delay value, i.e. timing data control data, for example by the method above. This timing delay control data is transmitted as part of the physical signal transmitted by the buffer 144 in the appropriate symbol slot as determined by the sequencer 146 with symbol boundaries synchronised to another version BCKM_T of the local clock BCKM_R and incorporating the sync symbols mentioned above to allow recovery at the slave of the slave receive clock BCKS_R.

In some embodiments, the timing delay control data transmitted from the master to slave may directly represent the delay to be applied within the slave. In other words, the controller 136 in the master may calculate delay value data to be stored in the slave register 184 to be directly applied to delay block 176. When this delay value data is received by the slave, the slave controller 170 may merely forward this delay value data unchanged to register 184, save for any necessary consistent data format or polarity or scaling operation.

In other embodiments, the timing delay control data generated and transmitted by the master may comprise merely an indication of the step size and polarity by which the applied delay needs to be increased or decreased, and the controller 170 in the slave may implement the necessary addition or subtraction.

In other embodiments, the timing delay control data generated and transmitted by the master may comprise merely an indication of whether the delay needs to be increased or decreased, and the controller 170 in the slave may implement a step size and polarity control algorithm (for example as described above) to provide a calculated revised value to register 184.

In still further embodiments, the control data transmitted by the master may simply indicate the timing of the transitions that it receives, and the slave controller may determine what delay should be applied based on this timing information.

In all cases however the applied delay is based on the transmitted timing delay control data.

FIG. 25 illustrates an alternative method of adjusting the delay value that is signalled from the master device to the slave device, in order that the signals from the slave device are received at the master device at the intended time when this delay is applied by the slave.

As described with reference to FIG. 23, the slave device sends a slave synchronisation data pattern to the master device. FIG. 24 shows an example where the synchronisation data pattern is made up of logical 0's and 1's, and the signal level 240 varies between the relevant levels. This method takes advantage of the fact that, in practice, the actual signal level does not change instantaneously from the logical 0 level to the logical 1 level, or vice versa. Thus, the master device samples the incoming signal sequence at some timing and aims to determine a measure for the difference in timing, T_Delta, between the sampling point T_Sample and the timing of the received signal transition, T_RX (that is, the midway point in the transition from the logical 0 level to the logical 1 level). It does this by using the actual sampled signal level in the range from 0 to 1. In this illustrated example, the sampling point occurs during a signal transition from the logical 0 level (L_Sym_0) to the logical 1 level (L_Sym_1). One advantage of using differential transmission over a balanced pair of wires is that the midway point in the transition from the logical 0 level to the logical 1 level is at zero volts.

It is assumed that, through some a priori knowledge, resulting from some preceding synchronisation mechanism, the timing of the transition is known to within +/−½ symbol period. FIG. 24 then shows actual sampled received signal level L_act being acquired at the sampling time T_Sample.

The timing of the received signal transition is defined by the moment at which the received signal crosses an expected signal level, L_exp, where said expected signal level is the average of the expected signal level during the first symbol of the two symbols forming the sync sequence, L_Sym_0 and the expected signal level during the second symbol of the sync sequence, L_Sym_1.

A simple measure for the difference in time can be derived from T_Delta_Estimate_Generic=Cs*Symbol_Period*(L_exp−L_act)/(L_Sym_1−L_Sym_0).

In this equation, Cs*Symbol_Period is the fraction of a symbol period over which the signal level transitions from the logical 0 level to the logical 1 level, and it can be assumed that Cs is constant. In addition, L_Sym_1 and L_Sym_0 are constant values, and estimates for both are known at the master device. As such, the combination of multiplication by (Cs*Symbol_Period) and division by (L_Sym_1−L_Sym_0) can be regarded as a multiplication operation by some constant C: T_Delta_Estimate_Generic=C*(L_exp−L_act)

In an actual implementation, sampled signal values are typically quantized with finite resolution as part of the sampling process and mapped on a finite set of discrete levels. Even when the quantization process only discerns as few as two discrete levels, each discrete level can be mapped backwards on an input signal level which represents the average signal level at the input of the sampler for all cases in which the sampled and quantized signal equals said discrete signal level.

Based on such inverse mapping operation an estimate for L_act, namely L_act_est, can at any moment be derived from any sampled and quantized signal value, sampled at T_sample.

Without changing the essence of said procedure, a measure for T_Delta can be formulated as: T_Delta_Estimate_Generic=C*(L_exp_L_act_est)

In practice, it is useful to sample the received signal multiple times, during different frames, in order to obtain multiple samples of the received signal, and these can be used to produce a more accurate value for L_act_est, mitigating the effect that noise may have on the timing or the level of the sampled signal. In general, this more accurate value can be obtained by calculating an average value L_act_est_Ave, of L_act_est by calculating the sum of N values of L_act_est and dividing that sum by N.

In a specific case in which the receiver maps the incoming received signal on logic 0 and 1, in which the first symbol of the synchronisation sequence corresponds to a logic 0 and the second symbol to a logic 1, a practical implementation may average 16 samples to obtain a single measure for T_Delta.

The average can be obtained by counting the number of logic 1s in a set of 16 samples L_act₀, . . . , L_act₁₅, to obtain a value Count1s, and dividing this number by 8. In this case L_exp=0.5 and (L_Sym_1−L_Sym_0)=1.

As a consequence, the estimate for the time difference can be obtained as. T_Delta_Estimate_0_1=C*(0.5−Count1s/16)

A further simplification can be obtained by some further scaling of the constant, C, effectively dividing it by 16. T_Delta_Estimate_0_1=C ₁₆(8−Count1s).

More generally, it is determined whether the timings of the signal level transitions as received at the master device are early or late relative to the desired timing by comparing a signal level at the desired timing with an expected signal level at the desired timing.

As shown in the example given in FIG. 24, the resulting estimate can be used to adjust the delay control data to either advance or retard the transmission of the symbols from the slave, and hence alter the timing of the transitions as detected by the master in the next iteration. The step size of such adjustments can be changed as described with reference to FIG. 24, or kept constant, or changed in any other way, in order to achieve the required combination of the speed with which the timing converges to the desired value and the precision of the convergence.

In cases where the sampled signal values are measured using a quantization process with more than two discrete levels, the resulting estimate can for example be used to adjust the delay control data to either advance or retard the transmission of the symbols from the slave by an amount that is proportional to the difference in the timing of the transitions from the desired timing.

A physical embodiment may be similar to that discussed with respect to FIG. 18. In a calibration mode, the sampling circuit 130 may generate multi-bit representations of the sampled signal and transmit these directly to the controller 136 via an additional signal path (not illustrated in FIG. 18).

After the master device has transmitted the timing delay control data to the slave (accessory), and the accessory has received the timing delay control data and updated its stored delay value accordingly, and after this process has iterated until a desired termination criterion is achieved, the master and slave enter a full synchronisation state as shown in step 198 of FIG. 19.

The master can now send to the slave details of the numbers of symbol slots allocated for transmission of payload data from the master to the slave (Nd) and from the slave to the master (Nu), if these have not previously been sent, data can then start being sent. Thus, the master device and the slave accessory enter the full operational mode, as illustrated in FIGS. 5 and 6.

FIG. 26 is a timing diagram, showing in more detail the symbols sent between the master device and the slave device in the full synchronisation state. In this illustrated frame, the control bit (shown as C↓ in FIG. 26) is being sent from the master device to the slave device. FIG. 26 can be read in conjunction with FIG. 18, which shows the generation of the relevant signals.

Thus, FIG. 26 shows:

-   -   the frame clock FCKM present in the master device, with one         pulse in each frame;     -   the ENM signal, for enabling the master device to transmit the         synchronisation data pattern, the downlink payload data, and (in         this frame) the control bit C↓,     -   the master reception clock BCKM_R, edges of which are used to         time the sampling of symbols transmitted to the master from the         slave (as shown in symbols D₁↑ and D₂↑ in FIG. 26,     -   the master transmission clock BCKM_T, edges of which are used to         time the generation of symbols for transmission from the master         to the slave,     -   the timing of signals on the bus, namely the synchronisation         data pattern (shown as 0↓ and 1↓ in FIG. 26), the control bit         C↓, the uplink payload data symbols D₁↑ . . . D_(Nu)↑, and the         downlink payload data symbols D₁↓ . . . D_(Nd)↓,     -   the slave reception clock BCKS_R, edges of which are used to         time the sampling of symbols transmitted to the slave from the         master (as shown in symbols D₁↓ and D_(Nd)↓ in FIG. 26,     -   the slave transmission clock BCKS_T, edges of which are used to         time the generation of symbols for transmission from the slave         to the master, and     -   the ENS signal, for enabling the slave device to transmit the         uplink payload data.

In this example, the delay t_(ds) configured by the master device in the slave device is such that the total round trip delay from the master to the slave slightly less than one symbol period. The total round trip delay is given by: t _(dRT) =t _(pD) +t _(pU) +t _(ds)

-   -   and this is shorter than one symbol period by the margin t_(m).

Thus, when the master device receives the last data pulse transmitted up from the slave, it has this margin, indicated by the reference numeral 250 in FIG. 26, before it is required to start the transmission of the first data pulse to the slave. This margin avoids problems caused by glitches and reflections at the transition.

This margin is provided by generating the clock BCKM_R timed slightly earlier than BCKM_T.

Similarly, the delay value t_(ds) is used to set the slave transmit clock BCKS_T at a timing that is offset from the recovered receive clock BCKS_R.

In one example embodiment, the frame rate is 1.536 MHz, and so the superframe rate is 1.536 MHz/128=12 kHz. The bit rate is 1.536 MHz×N_Symbol, so may for example be 1.536 MHz×20.

There is thus described a system that allows for the efficient transmission of data over a half-duplex wired communication link in an audio system, by compensating for the round-trip transmission delay between a host and an accessory in the audio system.

It is noted that the embodiments described above may be used in a range of devices and systems, including, but not limited to audio systems comprising a portable communications device and an audio accessory.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single feature or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope. 

The invention claimed is:
 1. A method of compensating for a round-trip transmission delay in an audio system comprising: a portable communications device; an audio accessory; and a cable, suitable for connecting the portable communications device and the audio accessory, having at least one wire and being detachable from at least one of the portable communications device and the audio accessory, the method comprising: at first times, transmitting on the at least one wire, from said device to said accessory, a first synchronisation data pattern, wherein said first synchronisation data pattern comprises first signal level transitions on the at least one wire, synchronised to a master transmission clock; at second times, transmitting, on the at least one wire, from said accessory to said device, a second synchronisation data pattern, wherein said second synchronisation data pattern comprises second signal level transitions at timings that are set based on a delay value stored in said accessory; in said device, determining, relative to the master transmission clock, timings of said second signal level transitions as received at said device; transmitting, from said device to said accessory, timing delay control data based on said determination; in said accessory, receiving said timing delay control data; and updating said stored delay value based on said timing delay control data.
 2. A method as claimed in claim 1, comprising: repeatedly iterating the steps of transmitting the second synchronisation data pattern, determining timings of said second signal level transitions, transmitting timing delay control data, receiving said timing delay control data, and updating said stored delay value based on said timing delay control data, until a termination condition is met.
 3. A method as claimed in claim 1, comprising: in said device, setting a desired timing relative to the master transmission clock; and determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing.
 4. A method as claimed in claim 3, comprising: transmitting, from said device to said accessory, timing delay control data to retard or advance the timings of the second signal level transitions at said accessory, depending on whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing.
 5. A method as claimed in claim 4, wherein said timing delay control data is adapted to retard or advance the timings of the second signal level transitions at said accessory by a known step size.
 6. A method as claimed in claim 5, wherein said known step size is varied from step to step.
 7. A method as claimed in claim 6, comprising halving said known step size each time that the determination as to whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing produces a result that is opposite to an immediately preceding iteration.
 8. A method as claimed in claim 3, comprising determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing a signal level at the desired timing with an expected signal level at the desired timing.
 9. A method as claimed in claim 8, comprising determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing an average signal level at the desired timing, obtained over multiple transitions, with the expected signal level at the desired timing.
 10. A method as claimed in claim 8, comprising determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by detecting whether signal levels at the desired timing represent logic 0's or logic 1's, and comparing the numbers of logic 0's or logic 1's obtained over multiple transitions.
 11. A method as claimed in claim 10, comprising: comparing a proportion of logic 0's and 1's with a 50:50 split.
 12. A method as claimed in claim 3, wherein said desired timing relative to the master transmission clock provides a predetermined time margin at the device between an end of a received symbol and a start of a transmitted symbol.
 13. A method as claimed in claim 1, further comprising, after performing said method, entering an operational mode.
 14. A method as claimed in claim 13, comprising, in the operational mode, in each of a plurality of frames: transmitting a synchronisation data pattern from said device to said accessory; transmitting first payload data from said device to said accessory; transmitting second payload data from the accessory to said device; and transferring control data between the device and the accessory, wherein transmissions from the device are synchronised to the master transmission clock, wherein transmissions from the accessory are at timings determined by the stored delay value, and wherein the format of the frame is such that, irrespective of whether the control data is transmitted from the device to the accessory or transmitted from the accessory to the device, there is only one pair of reversals of a direction of data transfer in each frame.
 15. A method as claimed in claim 14, wherein a plurality of said frames form a superframe, and wherein control data in the frames of a superframe forms a plurality of control words.
 16. A method as claimed in claim 1, wherein said timing delay control data transmitted from the device to the accessory directly represents the delay to be applied within the accessory.
 17. A method as claimed in claim 1, wherein said timing delay control data transmitted by the device comprises an indication of the step size and polarity by which the stored delay value needs to be increased or decreased.
 18. A method as claimed in claim 1, wherein said timing delay control data transmitted by the device comprises an indication of whether the delay needs to be increased or decreased.
 19. A method of compensating for a round-trip transmission delay in an audio system comprising: a portable communications device; an audio accessory; and a cable, suitable for connecting the portable communications device and the audio accessory, having at least one wire and being detachable from at least one of the portable communications device and the audio accessory, the method comprising, in the portable communications device: at first times, transmitting on the at least one wire first synchronisation data pattern, wherein said first synchronisation data pattern comprises first signal level transitions on the at least one wire, synchronised to a master transmission clock; at second times, receiving from said accessory a second synchronisation data pattern, wherein said second synchronisation data pattern comprises second signal level transitions; determining, relative to the master transmission clock, timings of said second signal level transitions as received at said device; and transmitting to said accessory timing delay control data based on said determination.
 20. A method as claimed in claim 19, comprising: repeatedly iterating the steps of receiving the second synchronisation data pattern, determining timings of said second signal level transitions, and transmitting timing delay control data, until a termination condition is met.
 21. A method as claimed in claim 19, comprising: setting a desired timing relative to the master transmission clock; and determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing.
 22. A method as claimed in claim 21, comprising: transmitting to said accessory timing delay control data to retard or advance the timings of the second signal level transitions transmitted from said accessory, depending on whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing.
 23. A method as claimed in claim 22, wherein said timing delay control data is adapted to retard or advance the timings of the second signal level transitions transmitted from said accessory by a known step size.
 24. A method as claimed in claim 23, wherein said known step size is varied from step to step.
 25. A method as claimed in claim 24, comprising halving said known step size each time that the determination as to whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing produces a result that is opposite to an immediately preceding iteration.
 26. A method as claimed in claim 21, comprising determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing a signal level at the desired timing with an expected signal level at the desired timing.
 27. A method as claimed in claim 26, comprising determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by comparing an average signal level at the desired timing, obtained over multiple transitions, with the expected signal level at the desired timing.
 28. A method as claimed in claim 26, comprising determining whether said timings of said second signal level transitions as received at said device are early or late relative to the desired timing by detecting whether signal levels at the desired timing represent logic 0's or logic 1's, and comparing the numbers of logic 0's or logic 1's obtained over multiple transitions.
 29. A method as claimed in claim 28, comprising: comparing a proportion of logic 0's and 1's with a 50:50 split.
 30. A method as claimed in claim 21, wherein said desired timing relative to the master transmission clock provides a predetermined time margin at the device between an end of a received symbol and a start of a transmitted symbol.
 31. A method as claimed in claim 19, further comprising, after performing said method, entering an operational mode.
 32. A method as claimed in claim 31, comprising, in the operational mode, in each of a plurality of frames: transmitting a synchronisation data pattern from said device to said accessory; transmitting first payload data from said device to said accessory; receiving second payload data from the accessory; and transferring control data between the device and the accessory, wherein transmissions from the device are synchronised to the master transmission clock, and wherein the format of the frame is such that, irrespective of whether the control data is transmitted from the device to the accessory or transmitted from the accessory to the device, there is only one pair of reversals of a direction of data transfer in each frame.
 33. A method as claimed in claim 32, wherein a plurality of said frames form a superframe, and wherein control data in the frames of a superframe forms a plurality of control words. 